1. Field of Invention
The present invention relates generally to block based image processing, and more specifically to reducing undesirable artifacts associated therewith by isolating and filtering the DC image.
2. Background of Invention
Reconstructed pictures that were encoded using block based picture coding (e.g., JPEG, MPEG2, MPEG4) often contain perceptually annoying blocking artifacts and/or edgy luminance transitions in the flat regions. For example, during a low bit rate MPEG coding in an application such as video conferencing, the reconstructed data often exhibits visible compression artifacts. Reconstructed picture quality can be enhanced by means of post-processing an image with a de-blocking low pass filter. Some known post-processing methods use coded bit stream characteristics to predict the amount of artifacts and to select the strength of the filter to use. However, it would be desirable for post-processing to be uncoupled from the decoding, so that the post-processing could be reused in any block based coding application. For post-processing to be uncoupled from the decoding, the post processing methodology could not use quantization scales or discrete cosine transform (DCT) domain data, which are available at decoding time and used by prior art methods.
Additionally, prior art methods apply post-processing to either pixels or DCT coefficients of eight by eight blocks. Pixel domain processing is computationally expensive, because of the size and complexity of the pixel array. Pixel processing also does not robustly preserve image detail. DCT domain processing requires computationally expensive DCT and IDCT operations. DCT domain processing also requires compressed picture data, and must be applied at the picture decoding stage. Furthermore, DC terms in the DCT domain are related to eight by eight blocks. Modifying DC terms of an eight by eight block generally does not achieve good detail preservation.
What is needed are reconstructed block based-picture post-processing methods, systems and computer program products that are uncoupled from picture decoding, and consequently do not use quantization scales or DCT domain data for mode decision and/or processing. Furthermore, it would be desirable for the post-processing to neither be applied to pixels nor to DCT coefficients of eight by eight blocks, as applying post-processing to either is computationally expensive, and does not adequately preserve image detail.